Don't want to argue but the original question was how to weigh a car 'at home'. The bathroom scale system does that. And deflection of the board has zero impact on the result as long as the middle of the board does not touch the floor. Link between the tire and scale can go between California and New York and back as long as it is, a) carrying the weight of the wheel and, b) transfers the weight to the scale. Shape of the member between those two points is immaterial to the result. Granted the accuracy is not the best with the cheapie scale but again, the criteria was 'at home'. In fact I have used the method many times to size out coil springs for MII front end conversions and it works great. Wouldn't want to sell gold bullion using it but for car work it gets you in the ball park.
I am cheaper than the rest of you. If the car is drivable, I throw in a few old newspapers, go to my recycling center, get the car weighed, toss out the newspapers, get the car re-weighed, take my $0.17 check for the paper and a free, pretty accurate weight slip. Such a deal!